Various existing piezoelectric transformer devices using piezoelectric transformer elements made of piezoelectric ceramic have been proposed as transformer devices that are used for power supply circuits such as inverters. In a piezoelectric transformer device as disclosed in the following Patent Document 1, a piezoelectric transformer element is arranged in a mounting hole provided in a printed wiring board. In the piezoelectric transformer device as disclosed in Patent Document 1, the piezoelectric transformer element is supported by a support securing member reaching a peripheral printed wiring board portion from the upper surface of the piezoelectric transformer element arranged in the mounting hole.
On the other hand, the following Patent Document 2 discloses a piezoelectric transformer device 1001 as illustrated in an exploded perspective view in FIG. 6. A piezoelectric transformer element 1003 is mounted on an insulating member 1002 having flexibility in the piezoelectric transformer device 1001. The insulating member 1002 is a flexible printed board having a wiring pattern for electrically connecting the piezoelectric transformer element 1003 to the outside. The wiring pattern is electrically connected to a surface electrode 1003a or the like of the piezoelectric transformer element 1003.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-36545.
Patent Document 2: Japanese Unexamined Patent Application Publication No. 11-121826.
In the piezoelectric transformer device as disclosed in Patent Document 1, the above-mentioned support securing member reaches the peripheral printed wiring board from the upper surface of the piezoelectric transformer element. Therefore, decrease in thickness of the piezoelectric transformer device is limited by the amount of thickness of the support securing member. Further, the piezoelectric transformer element and the printed wiring board are coupled by the support securing member. Due to this configuration, when temperature change occurs in the piezoelectric transformer device, mechanical stress with difference between a thermal expansion coefficient of the printed wiring board and a thermal expansion coefficient of the piezoelectric transformer element tends to be applied to coupling portions between the piezoelectric transformer element and the printed wiring board and the support securing member.
On the other hand, in the piezoelectric transformer device 1001 as disclosed in Patent Document 2, the piezoelectric transformer element 1003 is laminated and mounted on the insulating member 1002. The piezoelectric transformer device 1001 is supported by being mounted on a printed wiring board (not illustrated). Accordingly, decrease in thickness of the piezoelectric transformer device 1001 is limited by the amount of thickness of the printed wiring board. Further, in order to seal the piezoelectric transformer element 1003 from the periphery, a sealing member or a case member needs to be prepared additionally. This also makes it difficult to decrease the thickness of the piezoelectric transformer device 1001.